I used them extensively when I was racing. There is some special prep and machine work required to use them successfully. The deck needs to be cut for wire O-rings around the cylinders for starters and we ALWAYS went to head studs to get more even torque when tightening the heads down. Some guys even groove the water and oil passages for O-rings!!
Some older copper gaskets came with O-rings around the cylinder openings and were stamped like the steel gaskets around other openings. My uncle, who was a machinist made a pair of copper gaskets for his '57 Black Widow Chevy when the steel ones wouldn't stay sealed. That little fuelie 283 was BAD and he knew how to make it run!!
